This invention relates in general to the field of directional couplers, and in particular to microstrip directional couplers using capactive or inductive compensation.
Quadrature directional couplers consisting of parallel-coupled microstrip transmission lines are used extensively in microwave and millimeter-wave integrated hybrid monolithic circuits. In general, quadrature directional couplers can be used in any microwave or millimeter-wave subsystem with applications which include, among others, power sensing, combining, dividing, balanced mixing, amplifying, and antenna feed networks.
Because microstrip transmission lines have an inhomogeneous dielectric consisting of part dielectric and part air, odd and even mode phase velocities in the transmission lines are unequal. This inequality manifests itself in the coupler's poor directivity. It is well known that the directivity performance becomes worse as the coupling is decreased, or as the dielectric permittivity is increased.
There are several traditional methods of improving the directivity of such couplers, including adding an additional layer of dielectric over the conductors for symmetry, serrating the gap between the conductors, adding lumped capacitors at each end of the coupler, or selecting two or more different materials of different thicknesses and permittivities for the multilevel substrate. However, each of these methods is associated with particular disadvantages. For example, adding a slab of dielectric above the conductive path for symmetry adds material and introduces adhesive between the metallization and the substrate. Such a structure, which is not monolithic, may require handcrafting, or at least additional fabrication steps. Serrating the gap between the conductors does not produce a satisfactory or sufficient compensation for all values of the coupling. In addition, as is also the case for adding lumped capacitors at each end of the coupler, there is only a crude design method for determining appropriate compensation relies heavily on empirical means. None of these methods encompass an accurate solution for the compensation necessary to realize an ideal microstrip directional coupler.
For example, while the developed equations for determination of lumped capacitance to add at each end of the coupler are nearly true for tight coupling, the center frequency predicted is lower than desired. This result necessitates foreshortening the coupled section. Furthermore, for loosely coupled sections, the equations are no longer valid. A single capacitive compensation method for directional couplers has been proposed by Herbert W. Iwer in U.S. Pat. No. 4,216,446, but the disclosure does not instruct how to execute the design.
Thus, what is needed is a method which overcomes previous shortcomings and has associated with it a closed form solution for the compensating lumped capacitance and a new odd mode characteristic impedance necessary to realize an ideal microstrip directional coupler. The results need to be accurate for either tight or loosely-coupled sections. The method should result in embodiments for both antisymmetric and symmetric microstrip directional couplers with single inductive or capacitive compensation.